1. Field of the Invention
The present invention relates to a stack body formed by stacking a plurality of fuel cells in a stacking direction and a pair of end plates provided at both ends in the stacking direction. Each of the fuel cells is formed by stacking a membrane electrode assembly and a rectangular separator. The membrane electrode assembly includes a pair of electrodes and an electrolyte membrane interposed between the pair of electrodes. Long sides of the pair of end plates are fixed together by tightening members.
2. Description of the Related Art
For example, a solid polymer electrolyte fuel cell employs a membrane electrode assembly (MEA) which includes an anode, a cathode, and an electrolyte membrane interposed between the anode and the cathode. The electrolyte membrane is a solid polymer ion exchange membrane. The membrane electrode assembly and separators sandwiching the membrane electrode assembly make up a unit cell (power generation cell) for generating electricity. In use, generally, a predetermined number of unit cells are stacked together to form a fuel cell stack mounted in a vehicle or the like.
In this type of fuel cell stack, a suitable tightening load need to be applied in the stacking direction in order to achieve the desired power generation performance and sealing performance.
For example, as shown in FIG. 11, in a fuel cell stack disclosed in Japanese Laid-Open Patent Publication No. 2004-362940, a stack body formed by stacking a plurality of fuel cells 1001 are sandwiched between end plates 1002, 1003, and a plurality of tightening plates 1004 are provided to hold the stack body. The tightening plates 1004 contact sides of the stack body, and a bent portion 1004a is provided at least one of upper and lower positions of the stack body. The bent portion 1004a of the tightening plate 1004 is fixed to the end plates 1002, 1003 using bolts 1005 and nuts 1006.
In Japanese Laid-Open Patent Publication 2004-362940, four rectangular tightening plates 1004 are used, and bent portions 1004a as both ends of the tightening plate 1004 are fixed at the center of each side of the end plates 1002, 1003. Thus, stress can be easily concentrated at specific positions of the end plates 1002, 1003 in the tightening direction (stacking direction). In the structure, distortion or the like occurs in the end plates 1002, 1003, and the tightening force cannot be applied uniformly to the entire surface (in particular, the entire power generation surface) of the stack body.
In particular, in a fuel cell stack mounted in a vehicle, external loads (impacts) may be applied to the fuel cell stack during traveling of the vehicle or the like. Therefore, it is required to prevent positional displacement in a direction intersecting the stacking direction of the unit cells.
In this regard, for example, a fuel cell stack disclosed in Japanese Laid-Open Patent Publication No. 2009-070674 is known. As shown in FIG. 12, at one end of the fuel cell stack, an end plate 1011a is provided. An insulating plate 1012a is stacked on the end plate 1011a. A current collection plate 1013a is stacked on the insulating plate 1012a. A plurality of unit cells 1014 are stacked on the current collection plate 1013a. A current collection plate 1013b is stacked on the unit cells 1014. An insulating plate 1012b is stacked on the current collection plate 1013b. An end plate 1011b is stacked on the insulating plate 1012b. Tightening members 1015 each having a shape of rectangular column having a rectangular shape in cross section are fixed to four corners of the end plates 1011a, 1011b, so as to apply loads between the end plates 1011a, 1011b in the stacking direction.
The fuel cell stack includes four external binding members 1016a, 1016b, 1016c, and 1016d extending in the stacking direction. Recesses 1017a, 1017b, 1017c, and 1017d are formed at the center of each of the upper long side, lower long side, left short side, and the right short side of the end plates 1011a, 1011b, the insulating plates 1012a, 1012b, the current collection plates 1013a, 1013b, and the stacked unit cells 1014. The external binding members 1016a, 1016b, 1016c, and 1016d are fitted to the recesses 1017a, 1017b, 1017c, and 1017d. 
In Japanese Laid-Open Patent Publication No. 2009-070674, the four tightening members 1015 are fixed between the end plates 1011a and 1011b, and the four external binding members 1016a to 1016d are fixed to the end plates 1011a, 1011b. In the structure, the number of components is significantly increased, and the cost becomes high. Further, operation of assembling the fuel cell stack becomes laborious.